Employed for agricultural purposes for decades, tractors have a basic architecture that has not changed significantly since their conception. Still today modern tractors typically include an internal combustion engine mounted forwardly of a transaxle. The transaxle itself houses a transmission and the required drive mechanics to drive a pair of rear wheels attached thereto. The engine and transaxle together commonly provide the chassis for supporting other components, thereby dispensing for the need for heavy support frames. For example, a front axle is typically suspended below the engine compartment and a driver's cab may be suspended above the transaxle. Although variations have been proposed the majority of tractors in use today employ this architecture.
From low horsepower utility tractors through to high horsepower agricultural tractors, a power take off (PTO) shaft is usually provided to deliver torque to implements attached to the tractor. Traditionally, the PTO shaft was located at the rear of the vehicle but the introduction of front mounted implements has seen a growth in the number of tractors with front PTO shafts in addition. Implements which exploit the torque delivered by PTO shafts range from low-power fertilizer spreaders to power hungry cultivation equipment.
Driven by the demands of ever-stricter emissions regulation, attempts have been made to reduce harmful emissions from internal combustion engines whilst also improving the efficiency of power extraction from the engine. The use of electrical motors and generators as used in some hybrid systems in automotive cars has been considered. However, the power demand characteristics of a tractor are significantly different to that of an automotive car.
International patent application publication WO-2007/031396 discloses a drive system for an agricultural tractor wherein an internal combustion engine drives an electric generator. The power created by the generator is utilised to power a plurality of electric motors to propel the vehicle. The engine also drives, via a mechanical connection, a PTO shaft for transferring a drive torque to an attached implement.
It is an object of the invention to provide an improved hybrid power system for a tractor.